1. Field of Invention
This invention relates to an amplifier circuit for use with a sensor which requires only three wires for connection to a remote circuit. In particular, this invention allows for continuous process output measurement and continuous on line diagnosis of electrodes.
2. Description of Prior Art
Prior art systems such as the pH measuring system of U.S. Pat. No. 4,829,253, issued May 9, 1989, are capable of determining unknown values such as a voltage representation of the pH of a liquid and also of diagnosing electrode problems by checking the impedance of the sensor. The present invention allows for continuous on line measurement of both the process measurement and impedance.
The method used to determine sensor impedance in the prior art is also not as reliable or as accurate as the present invention's method. Prior art measures the decay time of the process output signal, or direct AC or DC impedance while the present invention measures area of the process signal output. This method provides for high accuracy measurement over the entire range for failed and functional electrodes.
Three wire pre-amplifier design is also known in the industry, but such design requires extensive bootstrap and bias circuitry for support. Not only have such systems involved unnecessary cost and complexity, the additional biasing circuitry requires more room and is less easily fitted into an electrochemical probe. Another major benefit of the present invention over the prior art is that the present invention does not require the use of a solution ground to reference the solution, thus reducing the number of electrodes needed, the size of the transmitter, and the cost.